The pilots viewed the convective activity and precipitation using weather applications on their mobile phones to determine the most viable route for the flight to Newark Liberty International Airport (KEWR), Newark, New Jersey.

The flight crew decided that the line of thunderstorms appeared to be fragmented enough, with sufficient openings, to allow them to deviate from their planned route (Figure 1) around the weather system. At 1845, the aircraft departed and deviated 50 nautical miles (nm) north-northeast of KGRR (Grand Rapids). After its initial deviation, the aircraft turned east, paralleling a line of thunderstorms south of its position. The aircraft was in visual meteorological conditions (VMC).

Figure 1. Depiction of weather at time of departure (1845) and the aircraft's planned flight route (dashed line) (Source: National Oceanic and Atmospheric Administration [NOAA], National Weather Service, with TSB annotations)

The aircraft initially climbed to flight level (FL)330, where it remained for 1 minute, and then climbed to its assigned cruise altitude of FL370, which is the maximum operating altitude of the aircraft. During the climb, the aircraft experienced light turbulence.

While flying eastbound, the crew used the on-board weather radar system to see the weather system ahead. As the aircraft was in VMC, the flight crew also visually identified openings between the thunderstorms ahead. As the flight progressed, the crew observed the thunderstorms billowing, closing the gaps in the route ahead.

Soon after reaching FL370, the aircraft began to experience increasing turbulence. The aircraft speed was 0.63 Mach. At this point the flight crew were communicating with Toronto Area Control Centre (ACC) and had appropriate clearance to deviate from their intended route to avoid the weather system.

At 1908, the flight crew communicated with the company dispatch via the aircraft communication and recording system to request a route through the weather system, stating that they needed help picking their way through the storms and that the gaps between storm cells were closing quickly.At approximately 1914, the dispatcher, viewing the company flight-following /weather software (Figure 2), suggested that the flight crew proceed directly to Chris Hadfield Airport (CYZR), Sarnia, Ontario, and then to Erie International Tom Ridge Field Airport (KERI), Erie, Pennsylvania. The crew then turned the aircraft southbound toward Sarnia.

Figure 2. Dispatcher's view of the aircraft's flight track at approximately 1915, showing the actual flight path (blue line) and weather. The weather depiction was generated by a base reflectivity radar source, using the lowest of the reflectivity angles. (Source: ExpressJet, with TSB annotations)

At 1914:55, the turbulence increased and became more intense over a period of 1 minute.At 1915:20, the management of the engines' thrust was switched to the CRZF mode, the throttle lever angle (TLA) was reduced from 76° to 61°, and the engine compressor speeds (N1) decelerated from 94% to 85% over a seven-second period.At 1915:27, the flight crew progressively increased the TLA from 61° to 74°, and the engines' N1 increased from 85% to 93%. During that period, speed increased to 0.76 Mach, and the intensity of the turbulence increased, with peak vertical acceleration values of 0.49g and 1.42g.At 1915:51, the TLA was reduced to 25°. Over the following 13 seconds, there was a corresponding deceleration in N1 speed from 93% to 60%. During this thirteen-second period, speed increased to 0.79 Mach, the turbulence became more intense with peak values of 0.41g and 1.7g, and the roll attitude became unsteady, with the aircraft banking at angles ranging between 45° left and 14° right.

Radar data obtained after the occurrence indicated that, at 1916, the aircraft was flying in a southerly direction, through a line of thunderstorms (Figure 3).

Figure 3. Flight path (dashed line), weather, and aircraft location at 1916. The weather depiction was generated by a composite reflectivity radar source, using the maximum reflectivity from all of the multiple elevation angles. (Source: National Oceanic and Atmospheric Administration [NOAA], National Weather Service, with TSB annotations)

The autopilot disengaged coincidentally with the peak vertical acceleration values at the moment when the aircraft exceeded 0.78 Mach. The aircraft climbed and exceeded the assigned cruise altitude of FL370 without air traffic control clearance.The flight crew re-engaged the autopilot; however, the aircraft continued to climb through FL374 with increased turbulence. Three seconds later, vertical acceleration hit peaks of 0.20g and 1.7g, and the autopilot disengaged again. The pitch attitude was −1.7°, the angle of attack fluctuated from 3.8° to −9.9°, the outside air temperature was −45.4 °C, the speed was decelerating through 0.68 Mach, and the aircraft continued to climb through FL375.

At 1916:19, the aircraft reached the peak of its altitude excursion, FL376, 600 feet above the aircraft's maximum operating altitude. At this point, the uncommanded right bank angle reached 45° and the turbulence momentary peak values were 0.44g and 1.6g. No significant elevator and trim deflections that would have contributed to the climb were commanded.

During the severe turbulence, both before and during the altitude excursion,the peak vertical acceleration values fluctuated rapidly within the range of 0.15g to 2.07g; the angle of attack fluctuated rapidly within the range of −11.9° and 3.8°; and the outside air temperature fluctuated between −51 °C and −45 °C.

At 1916:26, shortly after reaching FL376, the aircraft pitched to 11° nose down and rolled 30° to the right. The flight crew increased the thrust (advancing the TLA to 70°) and, using flight control inputs, commanded an additional right roll. As a result of the crew's additional roll inputs, the aircraft reached a 63° right bank angle. The pitch attitude increased to 13° nose down, and the aircraft reached a descent rate of 3100 feet per minute (fpm). The crew briefly applied left roll inputs, and the right bank angle decreased from 63° to 33°.At 1916:27, the ice-detection system indicated the presence of ice. The aircraft encountered severe icing conditions. The anti-ice systems were auto-commanded ON for the wing, stabilizer, and engine surfaces.At 1916:38, as the aircraft was descending through FL368, the rate of descent increased to 4400 fpm; the aircraft was in a 42° right bank. Using the flight controls, the flight crew applied right roll inputs. As a result, the right bank angle increased from 42° to 77°. The roll inputs by the flight crew during the uncommanded descent were contrary to known aircraft upset-recovery techniques. The correct technique is to roll the aircraft in the shortest direction to wings-level.The pitch attitude increased to 17° nose down. The aircraft's speed was accelerating through 0.76 Mach, and the severe turbulence persisted.

When the aircraft was descending through FL364, the flight crew applied left roll inputs, and the right angle of bank subsequently decreased to 26°. The descent rate was 7500 fpm, and the pitch angle was 24° nose down.Beginning at 1916:48, as the aircraft was descending through FL355, the flight crew reduced the thrust (decreasing the TLA from 70° to 25°) over a period of 6 seconds. During those 6 seconds, the turbulence lessened from severe to moderate, the aircraft's pitch attitude decreased to 5° nose down, and the descent rate decreased to 6900 fpm.

At 1917:02, while aircraft speed was 0.80 Mach, the autopilot was briefly engaged for 1 second before being disengaged. The aircraft attitude was stable, and the descent rate had decreased to 2700 fpm while passing through FL338. The turbulence remained moderate.At 1917:11, at FL336, the descent was arrested; the aircraft had lost approximately 4000 feet of altitude. Shortly afterward, the ice-detection indications stopped, and the aircraft began to climb.

During the period of loss of control, the aircraft had sustained an average turn rate of 200° per minute, which had changed its heading from 180° magnetic (M) to 240°M, and had reached a peak descent rate of 9300 fpm. Severe turbulence had persisted throughout this period of upset and loss of control. Toronto ACC repeatedly attempted to re-establish radio contact with the aircraft. The flight crew's only response was “stand by,” but this was said with a tone and volume that suggested that something of an urgent nature was occurring.

Over a period of 3 minutes, the aircraft had a varying climb rate before re-establishing a cruise altitude of FL370. The engines were producing thrust of between 85% and 95%. Turbulence changed from moderate to light, and the icing conditions ceased.As the aircraft climbed, the autopilot was momentarily engaged 3 times. In each instance, it became disengaged during moments of heightened turbulence. Icing conditions returned.

While on a heading of 240°M, the flight crew requested a heading from Toronto ACC to exit the weather system. Again, the tone of voice and volume suggested urgency. Toronto ACC suggested a southbound heading; the aircraft turned southbound and, shortly afterward, exited the weather system.The aircraft turned eastward, and the flight proceeded to its destination, KEWR. It did not experience any additional encounters with severe turbulence.